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... Baltic Sea is a brackish water area with a salinity range of about 0-10 psu. In the Baltic proper ( Fig.1) and the Gulf of Finland blooms of nitrogen fixing cyanobacteria are common. ...
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In this report, proposed new measures for the Baltic Sea Action Plan 2021 were analyzed in terms of their costs, effectiveness to reduce pressures and improve state and as their total sufficiency to reach good state. The cost effectiveness analysis for new measures analyzes the costs and effectiveness
of new measures separately, compares measures w...
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... Consequently, model evaluations of simulated diazotrophy in the Baltic Sea have mostly been limited to long-term averages or interannual variations of cumulative proxies indicative of standing stocks 43,44 . Our focus is on environmental conditions preceding bloom manifestations (as picked up by satellite) rather than on conditions during blooms (which were already investigated in the past 45,46 ). Essentially, this study compares trajectories of water parcels that develop blooms with those that don't in the Baltic Proper. ...
... We set out to find links between environmental conditions and the occurrences of cyanobacteria blooms. Foregoing studies focussed on conditions during the peak of the blooms 45 , or on the frequency of bloom occurrences over the years 46 . In contrast, our approach of combining high-resolution ocean circulation modelling, satellite data, in-situ observations and a back tracing algorithm made it possible to compare the history and trajectories of water parcels comprising a later bloom with the history of "non-blooming" parcels. ...
In late summer, massive blooms and surface scums of cyanobacteria emerge regularly in the Baltic Sea. The bacteria can produce toxins and add bioavailable nitrogen fixed from atmospheric nitrogen to an already over-fertilized system. This counteracts management efforts targeted at improving water quality. Despite their critical role, the controls on cyanobacteria blooms are not comprehensively understood yet. This limits the usability of models-based bloom forecasts and projections into our warming future. Here we add to the discussion by combining, for the first time, satellite estimates of cyanobacteria blooms with output of a high-resolution general ocean circulation model and in-situ nutrient observations. We retrace bloom origins and conditions by calculating the trajectories of respective water parcels backwards in time. In an attempt to identify drivers of bloom development, we find that blooms originate and manifest themselves predominantly offshore where conditions are more nutrient-depleted compared to more coastal environments.
... In most biogeochemical models such as ERGOM (Neumann, 2000) or BALTSEM (Savchuk, 2002) the excess phosphate that remains after the nitrate depletion in April is considered to fuel and limit the mid-summer cyanobacteria bloom. However, Nausch et al. (2008) and Karlson et al. (2010) have shown that the excess phosphate is not related to the intensity of cyanobacteria blooms. The data of Nausch et al. (2008) indicated that the excess phosphate is almost entirely consumed by mid-June and that also storage of the excess phosphate as particulate organic phosphorus and transfer to the mid-summer production does not occur. ...
Over the last 1,000 years, changing climate strongly influenced the ecosystem of coastal oceans such as the Baltic Sea. Sedimentary records revealed that changing temperatures could be linked to changing oxygen levels, spreading anoxic, oxygen-free areas in the Baltic Sea. However, the attribution of changing oxygen levels remains to be challenging. This work simulates a preindustrial period of 850 years, covering the Medieval Climate Anomaly (MCA) and the Little Ice Age using a coupled physical-biogeochemical model. We conduct a set of sensitivity studies that allow us to disentangle the contributions of different biogeochemical processes to increasing hypoxia during the last millennium. We find that the temperature-dependent mineralization rate is a key process contributing to hypoxia formation during the MCA. Faster mineralization enhances the vertical phosphorus flux leading to higher primary production. Our results question the hypothesis that increased cyanobacteria blooms are the reason for increased hypoxia in the Baltic Sea during the MCA. Moreover, the strong contribution of the mineralization rate suggests that the role of temperature-dependent mineralization in current projections should be revisited.
The combined effect of changing climate and changing nutrient loads from land due to altered land use, sewage water treatment and emissions was studied using a 3-dimensional high-resolution coupled physical-biogeochemical model for the Baltic Sea. Results suggest that global warming causes increased water temperatures and reduced sea ice cover, combined (eventually) with increased winter mean wind speeds and increased river runoff. The projected hydrographic changes could therefore have significant effects on the marine ecosystem. These changes may compete with nutrient load reductions-presently under discussion-that aim to improve the ecological status of the Baltic Sea. Targets that may be sufficient in the present climate might fail under future climate conditions. Using the model, we investigated 4 climate change scenarios and 3 nutrient load scenarios, ranging from a pessimistic 'business as usual' to the 'most optimistic' case (including the Baltic Sea Action Plan, BSAP). In addition, using cause-and-effect studies, we analyzed changing simulated nutrient cycles, oxygen concentrations, and phytoplankton concentrations. As model results for the northern part of the Baltic (Bothnian Bay and Bothnian Sea) are not reliable, we focus the analysis on the Baltic proper, including the Arkona, Bornholm and Gotland basins. The degree of nutrient reduction in nutrient-load reduction scenarios is likely to differ under a future climate, but actions of the BSAP will reduce phytoplankton concentrations also in the future climate. However, the sensitivity of non-linear responses to climate change depends on processes that are not well understood, with current understanding limited by modelling uncertainties (e.g. in the long-term functioning of Baltic Sea sediments as sources and sinks of nutrients).
